Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
The turbine stage of a conventional turbocharger comprises: a turbine housing defining a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined in the housing between facing radially extending walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chamber communicate such that pressurised exhaust gas admitted to the inlet flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine wheel. It is known to improve turbine performance by providing vanes in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Turbines of this kind may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to in line with varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine inlet is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level that ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.
In one known type of variable geometry turbine, an axially moveable wall member, generally referred to as a “nozzle ring”, defines one wall of the inlet passageway. The position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. Thus, for example, as exhaust gas flow through the turbine decreases, the inlet passageway width may be decreased to maintain the gas velocity and optimise turbine output. This arrangement differs from another type of variable geometry turbine in which a variable guide vane array comprises adjustable swing guide vanes arranged to pivot so as to open and close the inlet passageway.
The nozzle ring may be provided with vanes that extend into the inlet passageway and through slots provided in a “shroud” plate defining a fixed facing wall of the inlet passageway, the slots being designed to accommodate movement of the nozzle ring relative to the shroud. Alternatively, vanes may extend from the fixed facing wall and through slots provided in the nozzle ring.
Typically the nozzle ring may comprise a radially extending wall (defining one wall of the inlet passageway) and radially inner and outer axially extending walls or flanges that extend into an annular cavity behind the radial face of the nozzle ring. The cavity is formed in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing) and accommodates axial movement of the nozzle ring. The flanges may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. In one common arrangement the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator, which axially displaces the rods.
One example of a variable geometry turbocharger is disclosed in EP 0654587, which discloses a nozzle ring that is additionally provided with pressure balancing apertures through its radial wall. The pressure balancing apertures ensure that pressure within the nozzle ring cavity behind the nozzle ring is substantially equal to, but always slightly less than, the pressure applied to the nozzle ring face by gas flow through the inlet passageway. This ensures that there is only a small unidirectional force on the nozzle ring which aids accurate adjustment of the nozzle ring position, particularly when the nozzle ring is moved close to the opposing wall of the inlet to reduce the inlet passageway towards its minimum width.
In addition to the conventional control of a variable geometry turbocharger in an engine fired mode (in which fuel is supplied to the engine for combustion) to optimise gas flow, it is also known to take advantage of the facility to minimise the turbocharger inlet area to provide an engine braking function in an engine braking mode (in which no fuel is supplied for combustion) in which the inlet passageway is reduced to smaller areas compared to those in a normal engine fired mode operating range.
Engine brake systems of various forms are widely fitted to vehicle engine systems, in particular to compression ignition engines (diesel engines) used to power large vehicles such as trucks. The engine brake systems may be employed to enhance the effect of the conventional friction brakes acting on the vehicle wheels or, in some circumstances, may be used independently of the normal friction braking system, to control, for example, the downhill speed of a vehicle. With some engine brake systems, the brake is set to activate automatically when the engine throttle is closed (i.e. when the driver lifts his foot from the throttle pedal), and in others the engine brake may require manual activation by the driver, such as depression of a separate brake pedal.
In one form of conventional engine brake system an exhaust valve in the exhaust line is controlled to block partially the engine exhaust when braking is required. This produces an engine braking torque by generating a high backpressure that retards the engine by serving to increase the work done on the engine piston during the exhaust stroke. This braking effect is transmitted to the vehicle wheels through the vehicle drive chain. U.S. Pat. No. 4,526,004 discloses such an engine braking system for a turbocharged engine in which the exhaust valve is provided in the turbine housing of a fixed geometry turbocharger.
With a variable geometry turbine, it is not necessary to provide a separate exhaust valve. Rather, the turbine inlet passageway may simply be “closed” to a minimum flow area when braking is required. The level of braking may be modulated by control of the inlet passageway size by appropriate control of the axial position of the nozzle ring. In a “fully closed” position in an engine braking mode the nozzle ring may in some cases abut the facing wall of the inlet passage. In some exhaust brake systems known as decompression brake systems, an in-cylinder decompression valve arrangement is controlled to release compressed air from the engine cylinder into the exhaust system to release work done by the compression process. In such systems closure of the turbine inlet both increases back pressure and provides boost pressure to maximise compression work.
It is important to allow some exhaust gas flow through the engine during engine braking in order to prevent excessive heat generation in the engine cylinders. Thus there must be provision for at least a minimum leakage flow through the turbine when the nozzle ring is in a fully closed position in an engine braking mode. In addition, the high efficiency of modem variable geometry turbochargers can generate such high boost pressures even at small inlet widths that their use in an engine braking mode can be problematic as cylinder pressures can approach, or exceed, acceptable limits unless countermeasures are taken (or braking efficiency is sacrificed). This can be a particular problem with engine brake systems including a decompression braking arrangement.
An example of a variable geometry turbocharger which includes measures for preventing generation of excessive pressures in the engine cylinders when operated in an engine braking mode is disclosed in EP 1435434. This discloses a nozzle ring arrangement having bypass apertures that provide a bypass path that opens when the nozzle ring approaches a closed position to allow some exhaust gas to flow from the turbine inlet to the turbine wheel through the nozzle ring cavity thereby bypassing the inlet passageway. The bypass gas flow does less work on the turbine wheel than gas flowing through the inlet passageway so that with the bypass passageway open the turbine efficiency drops preventing excessive pressure generation within the engine cylinders. In addition, the bypass gas flow can provide, or contribute to, the minimum flow required to avoid excessive heat generation during engine braking.
It is also known to operate a variable geometry turbocharger in an engine fired mode so as to close the inlet passageway to a minimum width less than the smallest width appropriate for normal engine operating conditions in order to control exhaust gas temperature. The basic principle of operation in such an “exhaust gas heating mode” is to reduce the amount of airflow through the engine for a given fuel supply level (whilst maintaining sufficient airflow for combustion) in order to increase the exhaust gas temperature. This has particular application where a catalytic exhaust after-treatment system is present. In such a system performance is directly related to the temperature of the exhaust gas that passes through it. To achieve a desirable performance the exhaust gas temperature must be above a threshold temperature (typically lying in a range of about 250.degree. C. to 370.degree. C.) under all engine operating conditions and ambient conditions. Operation of the exhaust gas after-treatment system below the threshold temperature range will cause the system to build up undesirable accumulations which must be burnt off in a regeneration cycle to allow the system to return to designed performance levels. In addition, prolonged operation of the exhaust gas after-treatment system below the threshold temperature without regeneration will disable the system and cause the engine to become non-compliant with government exhaust emission regulations.
For the majority of the operating range of, for example, a diesel engine, the exhaust gas temperature will generally be above the required threshold temperature. However, in some conditions, such as light load conditions and/or cold ambient temperature conditions, the exhaust gas temperature can often fall below the threshold temperature. In such conditions the turbocharger can in principle be operated in the exhaust gas heating mode to reduce the turbine inlet passageway width with the aim of restricting airflow thereby reducing the airflow cooling effect and increasing exhaust gas temperature. However a potential problem with the operation of a modem efficient turbocharger in this way is that increased boost pressures achieved at small inlet widths can actually increase the airflow offsetting the effect of the restriction, thus reducing the heating effect and possibly preventing any significant heating at all.
The above problems with exhaust gas heating mode operation of a variable geometry turbocharger are addressed in US published patent application No. US2005/0060999A1. This teaches using the turbocharger nozzle ring arrangement of EP 1435434 (mentioned above) in an exhaust gas heating mode. The bypass gas path is arranged to open at inlet passageway widths smaller than those appropriate to normal fired mode operation conditions but which are appropriate to operation in an exhaust gas heating mode. As in braking mode, the bypass gas flow reduces turbine efficiency thus avoiding high boost pressures, which might otherwise counter the heating effect. In addition to the bypass gas path, pressure balancing apertures (as disclosed in EP 0654587, mentioned above) may be provided to aid control of the nozzle ring position in an exhaust gas heating mode.
Whether operated in an engine braking mode (with or without a decompression brake system) or an exhaust gas heating mode, control of the nozzle ring position at very small inlet widths can be problematic as there can be a rapid increase in the load on the nozzle ring as it approaches a closed position. Even with the provision of pressure balancing apertures as mentioned above there can be a tendency for the nozzle ring to “snap” shut as it approaches close to the opposing wall of the inlet. In addition it can require a very large force to open a nozzle ring, which abuts the opposing wall of the inlet when in a fully closed position. It can also be difficult to ensure that there is always an optimum minimum flow through the turbine when the nozzle ring is in a fully closed position.